dont drive it over 80km/hr, in such case ur suspension 'd give up pretty soon
inspect ur tyres watchfully, i hope suspension is pretty okay, not causing this harmonics. next time buy good tyres and pay special attention to expiry date as well as speed rating/loading capacity, that might help u.
btw have u checked wheels lug/nuts, make sure they r not loose. and go through alignment, camber adjustment.
general tyres are bad but still pretty okay to withstand 80km/hr speed. actually this is the basic thing in suspension designing, u encounter this 2 times, due to float of resonance from tyres to suspension. once around 80km/hr and once at ~140km/hr ( in case of alto), after that things are pretty smooth, dont take my words and try it ever. different vehicles have different threshold.